Hydrocyclones have for a number of years been commonly used in the beneficiation and dewatering of aqueous slurries. The popularity of hydrocyclones is due to the fact that there are no moving parts inside the cyclone. In essence a hydrocyclone comprises a frusto-conical vertical column, the larger end of which is attached to a feed box into which a tangential feed pipe communicates. An overflow box is positioned above the feed box and includes a liquids discharge pipe. The centre if the feed box includes a vortex finder and the base of the frusto-conical column terminates in a narrow opening known as a spigot or apex of the hydrocyclone. The aqueous slurry is forced at high pressure tangentially into the feed box, this causes rotation of the slurry within the feed box and frusto-conical column or feed box extension. Solids are flung to the wall of the conical column and spiral down to the spigot. The bulk of the liquid spirals upwards and leaves the cyclone through the discharge pipe via the vortex finder. The solids fraction leaving through the spigot are determined by the spigot to vortex finder relationship. The only solids to escape with the bulk of the water are the particles which are so fine that the entrainment or drag forces are not overcome by the centrifugal forces. Consequently for any given feed pressure/rotational speed there is a "cut size" at which the drag and centrifugal forces are in balance. Particles finer than this cut size are dragged with the bulk of the liquid through the vortex finder, and particles coarser than the cut size report to the spigot.
It is also known to adapt hydrocyclones of the kind described above by attaching at the spigot or apex a device called a fishtail. The purpose of the fishtail is to regulate the amount of water which is delivered from the hydrocyclone with the spigot discharge. Without the fishtail, the centrifugal action of the cyclone tends to produce a cylindrical air core in the centre of the feed box extension. The cylindrical air core causes an upward stream of water and air to the overflow box. When the solid content/pressure of the infeed vary the presence of air in the feed box extension tends to cause a substantial variation in the solid content of the discharge. The fishtail is attached to the spigot to ensure both control and consistent solid content of the discharge. The fishtail comprises a one way valve formed by soft rubber lips in a manner similar to a gas-mask. This valve allows periodic release of discharge solids but prevents entry of air, and thus effectively removes the cylindrical air core.
Since no air is drawn into the cyclone the discharge of the water from the overflow pipe causes a vacuum within the cyclone. When the infeed pressure reaches a certain level this vacuum can be so great as to cause upward movement of the solids from the non-return valve of the fishtail thereby cause the cyclone to choke. Consequently hydrocyclones incorporating fishtails are provided with a vent pipe to allow air to enter the overflow box to reduce the siphon effect and consequently the vacuum within the hydrocyclone. It is usual to provide an adjustable gate valve to vary and adjust the entry of air via the vent pipe. However, the gate valve only provides a single setting at any particular time and therefore this arrangement only works satisfactorily for a limited range of operation of the cyclone. When feed tonnages rise excessively the cyclone may well choke thereby requiring further adjustment of the gate valve to reduce the siphon and allow freer discharge from the spigot.
It is problems of this kind that have brought about the present invention.
The hydrocyclone of the kind set forth herein comprises a frusto-conical feedbox extension having attached at the large end a feedbox which includes a tangential feedpipe, an overflow box above the feedbox including an overflow pipe and a vent pipe, the narrow end of the feedbox extension having attached thereto a fishtail.
According to one aspect of the present invention there is provided a hydrocyclone of the kind set forth herein in which a pressure compensating valve is provided in the vent pipe, the operation of the valve being dependent on the pressure in the feed pipe of the hydrocyclone.
Preferably the pressure compensating valve comprises valve member biased to an open position against a diaphragm located in a pressure chamber, one side of the diaphragm being in fluid communication with the infeed pressure whereby variation of the infeed pressure partially or totally closes the valve member against an associated valve seat. Preferably the valve member is coupled to the diaphragm via a rod, and a coil spring co-axially surrounds the rod. In a preferred embodiment the spring rate of the coil spring is adjustable.
The valve member is preferably tapered to seat in a correspondingly tapered seat.